Besides cylinders utilizing fluid pressure such as pneumatic cylinders and hydraulic cylinders, electric cylinders are widely used in which a thread rod or a nut meshed with the nut or the thread rod is moved in the axial direction thereof by the rotation of the other. The nut and the thread rod of such an electric cylinder compose a ball thread in combination with balls and an arc-shaped thread groove is formed in each of the nut and the thread rod. The plural balls are filled in a line of a threaded path formed by meshing the nut with the thread rod. The ball rolls and circulates therein in association with the rotation of either one of them. Ball threads of this kind are widely employed.
Rolling of the balls for moving the thread rod or the nut in the axial direction thereof causes friction, which is rather small compared with the sliding friction caused in general trapezoidal threads and square threads. Accordingly, it is widely known that the movement in the axial direction of the thread rod or the nut is smooth and that high speed operation can be easily attained with high accuracy.
In the case where large thrust is required, however, high load is applied to one of the thread rod and the nut to cause the balls to compress an extremely limited part of the groove wall, forming an indent in the groove. Accordingly, the thread rod or the nut encounters difficulty or impossibility of smooth movement in the axial direction. This means inapplicability to those applied for high load application. The use of a nut and a thread rod formed of a squire thread or a trapezoidal thread would cope with high load application. However, the travel speed thereof to a target point is low under no load application.
In contrast, the thread rod and the nut with increased diameters may cope with high load application. Also, there are proposed constitution in which the number of combinations of ball threads and nuts meshed therewith is increased for coping with high load application at low speed operation and no load application at high speed operation (Patent Documents 1 and 2, for example).
However, these proposals fall within the general bounds and provide no radial means for solving the aforementioned problems. Also, the diameters of the thread rod and the nut and/or the number of combinations of the ball threads and the nuts must be increased even for the movement to a target point under no load application. This is futile and invites an increase in size and production cost.
Patent Document 1: Japanese Patent Application Laid Open Publication No. 9-271154A ([Claims] and FIG. 1)
Patent Document 2: Japanese Patent Application Laid Open Publication No. 10-70864A ([Claims] and FIG. 1)